benwtrent commented on PR #13124:
URL: https://github.com/apache/lucene/pull/13124#issuecomment-1961417895
OK, my head is spinning a bit trying to grok the side-effects of the CMS. I
think I understand most of it now.
Currently we have two adjustable parameters.
`maxThreadCount` dictates merge through put, or now many merges we have at a
time.
`mergeWorkerCount` dictates back pressure, or when we stop allowing merges
to be queued at all & block upstream indexing from continuing.
Throughout the life of a merge, it can become (un)paused numerous times,
this is controlled via the `RateLimitedIndexOutput` which only rate limits
WRITING results, not reading or anything else related to threads.
Now we want to add a new configuration, `maxMergeThreads`, which will
control merge latency by adding parallelism to each individual thread.
Since merge thread pausing only has to do with index output, I am not sure
we need to add any individual thread throttling other than what’s already
there. The Directory wrapper will pause/throttle all writing occurring for the
merge. This is acceptable even if the merge is using multiple threads.
I also think that small merges (<50MB) should never be allowed to run over
multiple threads. Similar to how we never throttle those because they are so
small, the benefit in latency reduction will be minuscule and we should reserve
the extra thread usage to larger merges.
What I am stuck on is on this:
- Should we use a common pool for all merges? Thus restricting the total
threads used by merging to be `maxThreadCount` + `maxMergeThreads`? This will
simplify the logic in the CMS significantly as a single task executor can be
used for all merges.
- Or should we use an individual executor/pool per merge? Thus restricting
total threads by `maxMergeThreads` * `maxMergeThreads` (or some fraction of
`maxMergeThreads`)? This could get interesting… How do we determine how many
threads each merge can get? Are we ok with creating a new task executor on
every larger merge and then closing it?
What do you think @jpountz?
One other thing we need to fix is this idea of “numWorkers” in HNSW. It
seems like it should just pick optimal slices given the number of vectors it
has (similar to multi-segment search stuff). Chunk itself into those slices and
then be at the behest of the task executor. What say you @zhaih ? It seems
weird to use `numWorkers` as a way to say “only use these many threads” when we
have no knowledge of how many threads the task executor actually has.
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